1. Field of the Invention
The invention pertains to industrial processes involving plasma treatment, in which parts of a plasma treating apparatus which are exposed to plasma are constructed of ceramic filled, plasma-useful polymer.
2. Background Art
While plasma is as old as the universe itself, it was first identified as “radiant matter”, by Sir William Crookes in 1879, and its nature first described by Sir J. J. Thompson in 1897. “Plasma”, the term coined by Irving Langmuir in 1928, is now considered a fourth state of matter, and is a gas in which a certain portion of particles are ionized. The presence of a non-negligible number of charged particles renders plasma, unlike an ordinary gas, electrically conductive. Plasma may be generated by natural sources such as lightning discharges, but industrially useful plasmas are generated by application of electric and/or magnetic fields. Plasmas generated at low pressures include glow discharge plasmas, capacitively and inductively coupled plasmas, and wave heated plasmas. Plasmas generated at atmospheric pressure include arc discharge, corona discharge, capacitive discharge, and dielectric barrier discharge.
Plasmas have become important industrially in many applications. For example, so-called “corona treatment”, which is technically treatment by dielectric barrier discharge, is widely used to alter the surface of thermoplastics by etching and surface functionalization to render the surfaces more acceptive of paints, metal coatings, and adhesives. More recently, plasmas have become very important in the processing of semiconductor wafers and in the fabrication of electronic devices on these wafers. One example of the use of plasma in such applications is Plasma-Enhanced CVD, or “PECVD”. In PECVD, a strong electric field ignites a plasma between two electrodes, one of which holds a substrate. The plasma cracks the bonds of the CVD process gas, thus enhancing the role of deposition onto the substrate. Silicon may be deposited from silane by PECVD, for example.
Plasma etching is another common process in semiconductor device fabrication. Here, the plasma produces chemically reactive species from process gases, which react with atoms of the substrate to create volatile species. Ion implant processing, used to create sublayers of different chemical makeup from that of the substrate by ion bombardment, is also a plasma process. These are but few of many examples.
Common to all these plasma processes is the generation of a highly energetic, aggressive, reactive, and corrosive plasma, generally under vacuum conditions. It is just these qualities which make plasma and plasma assisted processes highly efficient. Yet, as is readily imagined and well documented, these properties also take their toll on process equipment. Thus, the portions of the reactor which are exposed to plasma, whether directly or indirectly, and even unintentionally or unavoidably, must be chemically resistant. The low pressures common to many plasma treatments encourages loss of ions and low molecular weight species from components exposed to plasma.
Chemical resistance is necessary in all plasma treatment apparatuses, since erosion of the surface of an apparatus component not only can destroy its net shape, but can also cause chemical degradation which compromises properties such as strength and modulus. Because such processes are generally conducted in moderately high to high vacuum, undesirable outgassing of degradation products may occur. Finally, particulate matter may spall off from eroded surfaces, contaminating the surface of the substrate. In the processing of semiconductor devices, even particles in the low nanometer range can be fatal to the operation of a central processing unit, or memory units such as RAM, DRAM, or SDRAM.
For these reasons, consumable parts in plasma processing chambers have been traditionally made of high temperature inorganic materials such as quartz, fused silica, sapphire (fused alumina), or ceramics such as those prepared from silica, ceria, and alumina. Silicon carbide has also been used, as a coating or as a ceramic material per se, as has also boron nitride. Examples of such parts include assembly screws such as cap screws, wall liners, wafer passage liners, pin lifters, clamp rings, edge rings, a variety of inserts and shields, etc. Examples of such parts may be found in U.S. Pat. No. 6,165,276, and in numerous manufacturers' brochures, for example those from Technics, Tegal, Novellus, and Applied Materials, to name but a few. Due to the expense of the base material, plus the difficulties of machining these exceptionally hard and often brittle components, their cost is quite high. Moreover, they are often brittle and easily damaged.
Manufacturers have long sought to replace such consumable parts with parts of less expensive and more easily machinable materials, but without great success. Metals are generally prohibited, since even the smallest trace amounts of metals can severely compromise integrated circuit behavior. Thus, metals can only be used where there will be no erosion of the metal surface, or where the metal surface is coated with a ceramic material, an expensive process. Many refractory or ceramic materials are also unsuitable, due to contamination of semiconductor substrates with metals. Titanium dioxide is one such material.
U.S. Pat. No. 7,670,688, for example, discloses erosion resistant parts coated with oxides of yttrium, preferably yttria or yttrium aluminum garnet. U.S. Pat. No. 6,074,488 discloses a number of erosion resistant materials. U.S. Pat. No. 6,726,799 discloses the continued need to compensate for erosion of components, proposing an apparatus modification to raise the focus ring of a plasma etching apparatus to compensate for wear rather than constantly replacing the ring.
US 2002/0086545 discloses a corrosion resistant component of a plasma chamber including a liquid crystalline polymer (LCP). In particular, the LCP that was used is VECTRA® LCP, which is a registered trademark of Celanese AG, or ZENITE® LCP, which is a registered trademark of E.I. Du Pont de Nemours and Company. In one embodiment, the LCP includes one or more fillers such as glass or mineral fillers. Although LCPs are successfully used in a many high-end applications, they have the disadvantage of being comparatively costly. Moreover, the high stiffness of LCPs requires processing under high shear conditions to orient the crystal regions. This is particularly feasible for injection molding of small parts. In contrast, larger parts such as those used in plasma focus rings for example made by extrusion, compression molding and even injection molding typically are made using low shear conditions and as a result the high modulus properties are not developed.
More conventional, non-LCP type polymers would thus be an excellent choice for reactor internals, provided that they are capable of meeting the materials requirements. However, the fact that polymer surfaces have been plasma modified for decades contravenes the perceived usefulness of polymers. Moreover, because many plasma treatments involve exposure of internal parts to temperatures above 250° C., the choice of polymers is limited to the engineering thermoplastics such as polyether ketone (PEK), polyetheretherketone (PEEK), their variants such as PEKK, polysulfones, polyethersulfones (PES), polyetherimides (PEI), high temperature polyamides, polyamideimides (PAI), polybenzimidazole (PBI), perfluoroalkoxy polymer (PFA) and the like, and thermoset polymers such as those based on epoxy resins, bismaleimides, aromatic cyanates, and the like.
In order to be useful in a plasma apparatus, the polymer must not only meet the temperature requirements and possess adequate mechanical strength and modulus, but it must also be resistant to the plasma environment as well as being free of outgassing and generation of particulates, both as initially installed and after prolonged use.
Etching environments are in general the most severe in plasma processing. There are predominately four plasma-gas mixtures representative of commercial use. These are 100% O2 (typical of preclean etch); 95% CF4/5% O2 (silicon etch); 50% CHF3/25% HBr/12.5% O2/12.5% Cl2 (polysilicon glass etch); and 75% Cl2/25% HBr (main etch). Even without generation of plasma, the corrosive nature of many of these etching mixtures is readily apparent. Their activation by plasma renders them far more energetic and corrosive than the gas mixtures per se.
Attempts to replace plasma reactor parts with less expensive and more easily processed thermoplastics have been done, but for the most part have been disappointing. Prior to the present invention, the only polymers which have achieved limited success were polyimide polymers such as Vespel® SP-1 polyimide, a product of DuPont. However, the erosion rate in oxygen plasma is far higher than satisfactory. The industry has continued to seek for a thermoplastic material which can be used in plasma reactors under plasma exposure conditions.
US 2008/0236744 discloses various polymers as being “plasma resistant”. However, these polymers necessarily have to include conductive particles. The conductive particles make sure that the polymer coating is at the same potential of the chamber wall and thus plasma electrical discharges will not reach the wall, thus avoiding wall corrosion.